Advanced Tecnology Extended
This term can have several meanings, depending on the context:
(1) Motherboard form factor created by Intel in order to replace the old AT standard. It is the most used form factor nowadays. Its standard size measures 12" x 9.6" (30,5 cm x 24,4 cm). There are other ATX motherboard sizes: Mini ATX (11.2" x 8.2" or 28.4 cm x 20.8 cm), FlexATX (9" x 7.52" or 22.9 cm x 19.1 cm) and MicroATX (9.61" x 9.61" or less, 24.4 cm x 24.4 cm or less).
(2) Power supply standard used by ATX motherboards. ATX power supplies have two extra wires not present on AT power supplies: 3.3 V voltage and power-on. Because of the existence of the power-on wire it is possible to make the PC to turn itself off. ATX power supply uses a 20-pin plug while AT power supply uses two 6-pin plugs (or one 12-pin plug), thus it is not possible to directly install an ATX power supply on an AT motherboard and vice-versa. It is important to notice that some AT motherboards have ATX power connectors, allowing them to use ATX power supplies.
(3) Case standard used by ATX motherboards. AT motherboards can only be installed in ATX cases if the board has an ATX power plug. ATX cases have an opening on their back measuring 1.57" x 6.3" (4 cm x 16 cm) used to put the motherboard connectors outside the case. AT cases don't have this opening.